Worldwide availability and low cost of acrylonitrile, along with low cost commercial polymerization processes, make polyacrylonitriles a desirable raw material for extrusion of films with excellent gas barrier properties. However, acrylonitrile polymers are generally not considered thermoplastic or melt processable because the melting point (heat distortion temperature) is higher than the temperature at which they decompose. That is, when heated, acrylonitrile polymers composed of 90 weight percent or more of acrylonitrile monomer will char before the melt can be processed at temperatures commonly used in plastics processing equipment.
Attempts have been made to overcome this disadvantage by mixing polyacrylonitriles, especially acrylonitrile copolymers, with various resins having a high heat distortion temperature. Although, several compounds and blends containing acrylonitrile copolymers have been reported, such blends typically are based on acrylonitrile copolymers containing up to 80% acrylonitrile and 20% or more of olefinically unsaturated carboxylic acid esters and/or a rubbery component. These acrylonitrile copolymers are often referred to as “high nitrile” co-polymers. Materials such as these are commercially available under the tradename of Barex® from British Petroleum. Although it has been reported that films have been cast from blends of such “high nitrile” acrylonitrile copolymers and certain compatible thermoplastic polymers containing polar groups that were mixed in a mutual solvent such as dimethylformamide (DMF), these blends were not shown to be melt extrudable. Other compositions have been reported that contain blends of “high nitrile” copolymers with components such as acrylo-indene copolymers, hydrophobized polyamides 11 and 12, and the like. However, in each of the foregoing blended compositions, the “high nitrile” component is an acrylonitrile copolymer containing 70% to 80% or less of the acrylonitrile monomer. Melt extrudable polyacrylonitrile compositions plasticized with fugitive solvents have also been described; however, such compositions require removal and expensive recovery of the solvent or solvents.
The lack of thermostability of polyacrylonitriles has been of great concern since their first use in the manufacture of acrylic fibers. In particular, discoloration due to thermal decomposition is a major problem in thermoplastic blends containing substantial amounts of polyacrylonitriles. A broad spectrum of compounds has been used in an attempt to combat the yellow discoloration that occurs even in the solution spinning of fibers. These compounds have included salts of Ca, Sr, Mg, Mn and Al, and maleic acid, maleic acid anhydride, maleates, boron compounds, and the like.
Therefore, despite their low cost and excellent gas barrier properties, polyacrylonitriles have been underutilized as engineering materials. The use of polyacrylonitrile today is virtually limited to fiber production and the manufacture of acrylonitrile-butadiene-styrene (ABS) terpolymers and a few other copolymers.
Therefore, there is a need to provide compositions containing polyacrylonitrile that are thermoplastic and, thus, are melt extrudable. Such compositions would be useful as engineering materials with good oxygen and carbon dioxide barrier properties in the packaging industry, as well as blow molded containers for the food and beverage industries, and the like.